The commercial introduction of cellular radio telecommunications approximately five years ago is revolutionizing the telecommunications industry. The commercial interest in the cellular technique stems from its ability to enable high volume traffic to operate over the limited number of available radio channels. This is accomplished by dividing large geographical areas into smaller geographical areas, or cells. This permits the reuse of the same radio channels in different cells which are sufficiently separated spatially so as to avoid interference. Consequently, a large geographical area which previously had been limited to, for example, 7000 channels, and consequently, to 700 telephone calls, could now be divided into, for example, 70 cells, each one of which could use a channel set comprising 100 channels, without interference from adjacent cells which are using different 100-channel sets. Consequently, 7,000 telephone calls can now be made from the large geographical area which previously had been limited to 700 telephone calls. This new architecture is effected by introducing appropriate telephone switches which enable the system to maintain the integrity of each telephone call as the source of the telephone call moves from one of the cells to another of the cells. To accomplish this, the system assigns a new frequency to a mobile telephone as it moves from one call to another, and assigns appropriate resources to route the signal from the new cell to the switch itself. Clearly, sophisticated routing and switching equipment, including appropriate software, had to be designed and developed to implement this architecture.
As cellular radio becomes more popular, the cells become saturated due to the presence of more active subscribers within a given cell than there are available frequencies. However, cellular radio has within it an inherent technique for dealing with such an increase in subscribers. This technique is called cell splitting. In implementing cell splitting, the size of the cell is reduced, thereby once again bringing the number of active subscribers within each cell to a number less than or equal to the number of available frequencies. However, the explosive growth of demand for cellular radio makes it apparent that the cell splitting technique will soon become ineffective. This ineffectiveness is not associated with some inherent limitation on the size of the cells, but rather is associated with limitations of the switching machines, which, as the cells become smaller and smaller, have a greater demand placed on them because of the increased frequency with which active subscribers cross cell boundaries. Clearly, every time an active subscriber crosses a cell boundary the switching maching must hand over the call, i.e., assign a new frequency to the mobile terminal and assign resources to connect the signal from the new cell to the switch. Larger switching machines may be able to handle this increased traffic volume. However, they are much more expensive and may not be readily available because of the explosive demand for cellular radio apparatus. This invention is a new cellular radio architecture and infra-structure which permits rapid growth by distributing many of the swtiching and control functions into small modulator units which can be easily added to the system as it grows.
To understand the basic philosophy and operation of the inventive architecture, it is helpful to understand the basic underlying principles of the classic circuit switch, the packet switch, and the virtual circuit packet switch.
The classic circuit switch involves an architecture which dedicates resources, including transmission resources, to each call, for the duration of the call. In circuit switching architecture, as it is applied to cellular radio, when the caller crosses a call boundary the cellular switch must release the resources that had previously been assigned to the given call, and must dedicate new resources to the call, thereby performing many of the functions characteristic of terminating a call and establishing a new call. Essentially all of the comercialized cellular radio architectures are circuit switch architectures, and, because of the heavy burden that such architectures place on the switch, these architectures rapidly saturate the switches as the cells are split time and again to meet increasing demand. Cell splitting increases the burden on the switch because as the cells are split the frequency of boundary crossing increases. (It is understood that the term "call" as used here, and the invention in general, is not limited to transmissions representing audio communications, but rather includes any type of communication including the transmission of data, facsimile, audio, video, etc.)
The pure packet switch architecture in some sense is the direct opposite of the circuit switch architecture, in that the pure packet switch architecture never permanently assigns transmission resources to a given call. Rather, the information being transmitted in the call is divided into packets of information, each one of which is assigned transmission resources based on "header" information associated with each packet, and is routed independently of the other packets of information. Clearly, the advantage of the packet switch architecture is that since transmission resources are not dedicated to any given call, such resources may be used for other calls when the information transmitted by a call is "bursty" rather than steady. The transmission resources can be used for other calls during the idle time periods between the bursts of information. The disadvantage of the packet switch architecture is that it places a heavy burden on the switch, since the switch must establish routing for each packet, unlike the circuit switch architecture which establishes "permanent" routing once for each call. The advantage of packet switching comes at the expense of bandwidth since each packet must contain the necessary "header" information which conveys to the switch the routing requirements of each packet.
In a sense, the advantages of the packet switch architecture and the circuit switch architecture are combined in the virtual circuit packet switch. In the virtual circuit packet switch architecture, the virtual circuit packet switch, like the circuit switch, establishes a route for each call at the beginning of the call. This route is stored in the memory of the switch. Thereafter, each packet contains in its header a call identification number, rather than more complete header information, which enables the switch to find in its memory the correct route for that call. In this manner, the switch does not have to go through the entire routing procedure in order to transmit each packet of a given call, but rather can rely on the previous routing procedure that had been established at the inception of the call. Although virtual circuit packet switching may be applied to cellular radio, switching must still occur as the active subscriber crosses each cell boundary. Consequently, in a very real sense, conventional virtual circuit packet switching offers little relief to the switch from the burdens associated with the increasing frequency of boundary crossings as the cells become smaller.